With soaring resource price, Cu has come to be used instead of Au as a material for bonding wires. As a bonding wire using Cu as a material, a Pd-coated Cu bonding wire is known, where the Pd-coated Cu bonding wire is produced by coating a core material mainly composed of Cu with Pd (e.g., Patent Literature 1).
A thermosonic bonding process is used commonly to join a bonding wire. This process involves passing the bonding wire through a jig called a capillary, heat-melting a wire tip by arc heat input, forming a free air ball (hereinafter also referred to simply as a “ball portion” or “FAB”) by surface tension, then pressure-bonding the ball portion onto an electrode of a semiconductor element heated in a range of 150 to 300° C., and then bonding the bonding wire directly on the side of an external lead by ultrasonic bonding.
In forming a ball portion on the tip of the Pd-coated Cu bonding wire, it is necessary to use a non-oxidizing atmosphere to prevent oxidation of the ball portion. Specifically, it is common practice to form the ball portion in non-oxidizing atmosphere gas made up of pure nitrogen, argon, or nitrogen containing approximately 5 vol % of hydrogen (e.g., Patent Literatures 2 to 5).